Poly-methyl-methacrylate (PMMA) bone cement has been in use since about 1960 for hip replacement surgery and not long thereafter came into use for percutaneous vertebroplasty, the latter being a palliative procedure requiring the injection of bone cement into the vertebral body at the cervical, thoracic or lumbar locations. The indications for percutaneous vertebroplasty are severe osteoporosis with vertebral compression fractures and vertebral haemangiomas and possibly patients with vertebral tumors. PMMA cement is manually injected into the vertebral body, the cement usually containing a high concentration of zirconium dioxide to allow for X-ray fluoroscopy. The cement permeates the vertebral body hardening and stabilizing the bony structure, the surgical procedure intending to stabilize the affected site and provide relief from significant pain.
PMMA is dough-like cement that gradually hardens into a solid material with good biocompatibility. The preparation of PMMA bone cement requires the combination of two components: a solid powder and a liquid monomer. The cement becomes progressively viscous as polymerization to poly-methyl-methacrylate proceeds at a rate governed by the Arrhenius equation. Specific clinical applications such as vertebral fracture augmentation (e.g. kyphoplasty, vertebroplasty, arcuplasty) demand an optimal range of viscosity. Upon mixing the two components, the latency to achieve usable viscosity is dependent on the ambient temperature. In clinical use it is often difficult to accurately anticipate the appropriate time for mixing of the PMMA. Consequently, it is frequent to wait for adequate polymerization before proceeding. Conversely, occasionally the PMMA will be too viscous to apply and will need to be discarded. A need exists in the art to adequately control the polymerization process and the viscosity of delivered PMMA in clinical orthopedic applications.
The present invention incorporates a solid-state Peltier junction with a PMMA reservoir on the cold side to prevent premature polymerization. As the PMMA is needed it is passed over the heated (opposite) side to provide adequate activation energy to ensure adequate polymerization as the PMMA exits the apparatus. A roller pump is integrated into the device.
Arrhenius equation may be utilized to predict cement activation and viscosity. As known in the art, Arrhenius equation is an expression that shows the dependence of the rate constant k of chemical reactions on the temperature T (in Kelvin) and activation energy Ea, according to:k=Ae−Eα/RT.where A is the pre-exponential factor or simply the prefactor and R is the gas constant. The units of the pre-exponential factor are identical to those of the rate constant and will vary depending on the order of the reaction. If the reaction is first order it has the units s−1, and for that reason it is often called the frequency factor or attempt frequency of the reaction. When the activation energy is given in molecular units, instead of molar units, e.g. joules per molecule instead of joules per mol, the Boltzmann constant is used instead of the gas constant. It can be seen that either increasing the temperature or decreasing the activation energy (for example through the use of catalysts) will result in an increase in rate of reaction.
Given the small temperature range in which kinetic studies are carried, it is reasonable to approximate the activation energy as being independent of temperature. Similarly, under a wide range of practical conditions, the weak temperature dependence of the pre-exponential factor is negligible compared to the temperature dependence of the exponential factor, exp(−Eα/RT); except in the case of “barrierless” diffusion-limited reactions, in which case the pre-exponential factor is dominant and is directly observable.
When a reaction has a rate constant which obeys the Arrhenius equation, a plot of ln(k) versus 1/T gives a straight line, whose slope and intercept can be used to determine Eα and A. This procedure has become so common in the art of chemical kinetics that practitioners often use it to define the activation energy for a reaction. That is the activation energy is defined to be (−R) times the slope of a plot of ln(k) vs. (1/T) at constant pressure P:
      E    a    ≡      -                  R        ⁡                  (                                                    ∂                ln                            ⁢                                                          ⁢              k                                      ∂                              (                                  1                  /                  T                                )                                              )                    P      
Once the activation energy Ea is determined for a given reaction involving PMMA cement, the viscosity may be predicted as a function of temperature and reaction time as known in the art. Furthermore, PMMA cement may be mixed with a chemical additive which predictably changes color with temperature as shown by D. C. Smith and M. E. D Bains, J. D. Res, Vol 35, No. 1, p 16-24. A bone cement dispensing device that controls PMMA cement temperature and uses a color based temperature indicator would be useful for delivering PMMA cement at a desired viscosity, temperature and polymerization rate to the desired bone location for proper setup. It is the objective of the present invention to provide such a bone cement dispensing device.